This invention relates generally to methods and apparatus for controlling the amount of reactant to be added to a substance to reduce such substance. More particularly the invention relates to methods and apparatus for reducing NOx with urea using a using a sensor which is responsive to un-reacted portions of the NOx and un-reacted portions of the urea.
As is known in the art, in many applications it is desirable to detect the effectiveness of a reaction used to reduce a substance. One such application is in measuring the effectiveness in urea based selective catalytic reduction (SCR) in reducing nitrogen (NOx) in the exhaust gas of a diesel engine. More particularly, an aqueous solution of urea is injected into the exhaust gas of the engine upstream of a catalyst. In order for the method to reduce NOx in the exhaust effectively, it is important that the amount of urea injected into the exhaust be accurately controlled. Injection of too little urea may result in sub-optimal (i.e., incomplete) NOx conversion. Injection of too much urea may produce nitrates in the exhaust which can reduce the life of the exhaust system downstream of the catalyst, may produce an unpleasant odor, and may also produce increases in regulated emissions.
Thus, it is desirable to have a sensor downstream of the catalyst which can detect the presence of NOx after the reaction. Unfortunately, currently available sensors which are practical for automotive use from cost and size perspectives cannot differentiate between NOx and urea. It has also been suggested that a sensor be placed upstream of the both the catalyst and the injected urea. Thus, with such an arrangement, the sensor would not be exposed to the urea and would only be responsive to the NOx in the engine exhaust. With this arrangement, however, a priori knowledge of the amount of urea to be injected into the engine exhaust for a measured amount of pre-reacted NOx is assumed. Thus, if the sensor produces a signal, nox1, for the urea injector, the correct amount of urea to be added is k_base*nox1, where k_base is an a priori determined factor. The factor k_base may be a function of several variables, such as engine speed and load etc. Thus, values of the factor k_base as a function of these variables may be stored in a look-up table. During operation, these variables are measured and are fed to the look-up table to provide the factor k_base. This arrangement, however, assumes that the factor k_base will not change with age, that the injector which is responsive to the signal, k_base*nox1, has a known, time invariant transfer function, k_injector, and that the catalyst is operating properly. Thus, this is a feed-forward control system.
In order to account for such variables as age, changes in k_injector, and catalyst efficiency variations, a feed-back loop control system which responds to the actual amount of NOx produced after the catalyst would be desirable. Unfortunately, as noted above, practical automotive NOx sensors are responsive to both NOx and urea.
As will be described in detail below, a detection method is disclosed which enables differentiation between the presence of urea and NOx by a sensor which is itself unable to differentiate between urea and NOx and to provide therefrom a control signal to optimize the injection of the reactant into the substance.
In accordance with the invention, a method is provided for controlling an amount of a first substance to be added to a reaction with at least a second substance of the reaction. The method is used to reduce the first substances of the reaction. The product of such reaction along with un-reacted portions of the substances are directed to a sensor. The sensor producing an output signal in response to detection of un-reacted portions of the first substance and un-reacted portions of the second substance. The method includes changing the amount of the first substance added to the reaction. The output signal is measured to determine whether such output signal increases or decreases with the changed amount of the first substance. From such measurement, a determination is made whether the sensor is responding to the un-reacted portions of the first substance or to the un-reacted portions of the second substance. The amount of the first substance to the reaction is adjusted in accordance with the determination.
In accordance with one embodiment, a method is provided for controlling an amount of a reactant used to reduce a substance added to such substance to react with such substance. The product of the reaction along with un-reacted portions of the substance and un-reacted portions of the reactant are directed to a sensor. The sensor produces an output signal in response to detection of both the un-reacted portions substance and the un-reacted portions of the reactant. The method includes changing the amount of reactant added to the substance in accordance with a measurement made to determine whether the output signal increases or decreases with the changed amount of reactant. More particularly, from such measurement, a determination is made whether the sensor is responding to the un-reacted reactant or to the un-reacted substance and the amount of the reactant added to the substance is adjusted in accordance with the determination.
In accordance with one embodiment, the reactant is urea and the substance is NOx.
Thus, with such an arrangement, a NOx, urea-nonselective, sensor may be placed downstream of the catalyst. Consequently, adjustments in the urea may be made because of aging effects and changing ambient conditions. Thus, the amount of NOx actually produced may deviate from the predicted quantity. To put it another way, a system which merely measures engine speed and load, gas temperature, and rate of acceleration etc. to make an open-loop determination of the amount of urea to be added to the engine exhaust may not adequately compensate for the aging effects and changing ambient conditions. Here, with the sensor and processing method, these deviations from any open-loop predicated amount will automatically be compensated in a feedback arrangement which properly controls the amount of urea to be injected to provide stoichiometry.
In accordance with one feature of the invention, a method is provided for controlling an amount of a reactant used to reduce a substance added to such substance to react with such substance. The product of such reaction along with un-reacted portions of the substance and un-reacted portions of the reactant are directed to a sensor. The sensor produces an output signal in response to detection of both the un-reacted portions substance and the un-reacted portions of the reactant. The method includes periodically increasing and decreasing the amount of reactant added into the substance. Changes in the output signal produced by the sensor are caused by such periodical increases and decreases. The method includes integrating over each period a mathematical product of the periodically increasing and decreasing the amount of reactant with the measured change in output signal to produce a control signal. The amount of the reactant added to the substance is adjusted in accordance with the control signal.
In accordance with another feature of the invention, a processor is provided for controlling the addition of a reactant to a substance to react with such substance. The product of such reaction along with un-reacted portions of the substance and un-reacted portions of the reactant are directed to a sensor. The sensor produces an output signal in response to detection of both the un-reacted portions of the substance and the un-reacted portions of the reactant. The processor changes the amount of reactant added into the substance in accordance with a program to enable the processor to determine whether the change in the amount of reactant and the change the output signal are in the same direction or in opposite directions.